Scanning optical system in which a ghost image is eliminated

ABSTRACT

A scanning optical system which is provided with a deflector for deflecting a light beam, a medium to be scanned by the light beam deflected by the deflector, and an imaging optical system disposed between the medium to be scanned and the deflector. A linear image near the deflecting - reflecting surface of the deflector and a point on the surface of the medium to be scanned are in a conjugate relation through the imaging optical system. The imaging optical system is such that the angle which the optical axis of the imaging optical system forms with respect to the optical axis of the light beam incident on the deflector in a plane parallel to the deflecting plane of the light beam is chosen so that a ghost image is formed in the direction of the scanning line of the medium to be scanned and outside the effective scanning area.

This application is a continuation of application Ser. No. 724,044filed, Apr. 18, 1985, now abandoned, which was a continuation ofapplication Ser. No. 434,331, filed Oct. 14, 1982, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a scanning optical system in which a ghostimage is eliminated.

2. Description of the prior art

In recent years, a technique of scanning the surface of a photosensitivemedium with a modulated light beam to effect recording has often beenused. It is known that when a surface to be scanned is scanned by adeflector using a plurality of reflecting surfaces such as a polygonmirror, the scattered light beam created on the surface to be scannedcan become a ghost image and imparts an undesirable influence to theimage. Means for eliminating such ghost image are disclosed in U.S. Pat.No. 4,040,737. The technique thereof is shown in FIG. 1 of theaccompanying drawings. If a scanning optical system is used in which aparallel light beam Lc impinges on the reflecting surface 3a of adeflector 3 and the deflected light beam Ld is imaged on a medium 6 tobe scanned by a rotation-symmetric optical system 7 to cause the lightbeam Lc to further impinge at an angle relative to a plane perpendicularto the rotational axis 8 of the deflector 3, elimination of the ghostbecomes possible. That is, by a slit 9 being disposed between theoptical system 7 and the medium 6 to be scanned and adjacent to themedium 6 to be scanned, the ghost image Pg formed in the directionorthogonal to a scanning line 10 can be intercepted.

In the field of such a scanning optical system, there is known a systemwhich prevents the position of the scanning line on the surface to bescanned from being varied by the falling of the deflecting-reflectingsurface of the deflector or the falling of the rotational axis of thedeflector. FIG. 2 of the accompanying drawings shows an example of theconstruction of such scanning optical system. A light beam L emittedfrom a light source device 1 comprising a light source, a condenser,etc. passes through a linear image forming system 2 such as acylindrical lens and impinges on a reflecting surface 3a of a deflector3 comprising a rotatable polygon mirror while being linearly converged.The light beam L is reflected by the reflecting surface 3a, passesthrough an imaging optical system comprising a spherical single lens 4and a single lens 5 having a toric surface having a major axis and aminor axis having different refractive powers in two orthogonaldirections and impinges on a medium 6 to be scanned, thus forming animaged spot thereon. This imaged spot scans the medium 6 to be scannedat a predetermined speed with the rotation of the deflector 3.

FIG. 3 of the accompanying drawings shows the optical path in a crosssection parallel to the deflecting surface of the above-describedconstruction, in other words, a plane containing the major axis of thesingle lens 5 and the optical axis of the spherical single lens 4. FIG.4 of the accompanying drawings shows the optical path in a directionperpendicular to the deflecting plane of deflection of the light beam Las deflected by the deflector 3, and illustrates the influence of thefalling of the reflecting surface 3a of the deflector 3. The light beamL emitted from the light source device 1 is linearly imaged near thereflecting surface 3a of the deflector 3 by the linear image formingsystem 2. The refractive power of the single lens 5 in the cross sectionof FIG. 4 differs from the refractive power of the single lens 5 in thedeflecting plane of FIG. 3, and in the imaging optical system comprisingthe spherical single lens 4 and the single lens 5, the positionalrelation between the reflecting surface 3a of the deflector 3 and themedium 6 to be scanned is an optically conjugate relation. Accordingly,even if the reflecting surface 3a is inclined from a directionperpendicular to the deflecting plane during rotation of the deflector 3and changes to a position 3A, the light beam L passing through theimaging optical system comprising the single lenses 4 and 5 changes asindicated by dotted lines but yet no change of the imaged positionthereof on the medium 6 to be scanned occurs.

Again in such a scanning optical system, as shown in FIG. 5 of theaccompanying drawings, the light beam L having impinged on a point Ps onthe medium 6 to be scanned is diffusion-reflected on the surface of themedium 6 to be scanned, and the reflected light La thereof passesthrough the single lenses 5 and 4 and again impinges on the deflector 3,as indicated by dotted lines. At this time, the reflected light La fromthe medium 6 to be scanned which has impinged on the reflecting surface3a is reflected toward the light source device 1 side, while part of thereflected light La from the medium 6 to be scanned impinges on areflecting surface 3b adjacent to the reflecting surface 3a and is againreflected and passes through the single lenses 4 and 5. The light beamLb concentrates in the vicinity of the point Pg on the medium 6 to bescanned. This light beam Lb becomes a ghost image and, if aphotosensitive medium is installed on the medium 6 to be scanned, therewill be formed an undesirable image.

In such a scanning optical system wherein the falling of thedeflecting-reflecting surface is corrected, the linear image near thedeflecting-reflecting surface 3a and the point on the surface of themedium 6 to be scanned are in a conjugate relation as shown in FIG. 4and therefore, even if the incident light beam L is inclined relative tothe rotational axis of the deflector as shown in FIG. 5, there is aproblem that a ghost image is formed on the same scanning line.

SUMMARY OF THE INVENTION

It is an object of the present invention to eliminate the above-notedproblem in a scanning optical system wherein correction of falling hasbeen made and to provide a scanning optical system in which a ghostimage is always caused to be formed at the same position outside thescanning line independently of the rotation of a deflector, whereby theghost image is eliminated.

The scanning optical system according to the present invention isprovided with a deflector for deflecting the light beam from a lightsource unit and scanning a medium to be scanned, and an imaging opticalsystem disposed between the medium to be scanned and the deflector. Alinear image near the deflecting-reflecting surface of the deflector anda point on the surface of the medium to be scanned are in a conjugaterelation through the imaging optical system. The imaging optical systemis such that the angle which the optical axis of the imaging opticalsystem forms with respect to the optical axis of a light beam incidenton the deflector in a plane parallel to the deflecting plane of thelight beam is so selected that a ghost image is formed in the directionof the scanning line of the medium to be scanned and outside theeffective scanning area.

The invention will become more fully apparent from the followingdetailed description thereof taken in conjuncion with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a scanning optical system according to the prior art inwhich a ghost image is eliminated.

FIG. 2 shows an embodiment of the scanning optical system according tothe prior art which has a falling correcting function.

FIG. 3 shows an optical path in a plane parallel to the deflecting planeof the scanning optical system shown in FIG. 2.

FIG. 4 shows an optical path in a plane perpendicular to the deflectingplane of the scanning optical system shown in FIG. 2.

FIG. 5 illustrates the manner in which a ghost image is created in thescanning optical system shown in FIG. 2.

FIG. 6 illustrates the scanning optical system according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention will hereinafter be described in detail with respect to anembodiment thereof shown in FIG. 6. In FIG. 6, reference numeralsidentical to those in FIGS. 1 to 5 designate identical elements. In FIG.6, a light beam L emitted from a light source device 1 impinges on adeflector 3 at an angle α with respect to the optical axis C of animaging optical system 20 comprising single lenses 4 and 5. In thedeflecting plane of the light beam L, the imaging optical system 20 hasthe f.θ characteristic that the distance from the optical axis C of theimaging optical system 20 to an imaged spot Ps is proportional to adeflection angle θ which the principal light ray reflected by thereflecting surface 3a of the deflector 3 forms with the optical axis Cof the imaging optical system 20. In a plane parallel to the deflectingplane of the light beam L and containing the optical axis C of theimaging optical system 20, the direction of the light beam L from thelight source device 1 is chosen so as to satisfy the following relation:

    |α|<(4π/N)-(W/D)

where D is the distance from the image side principal point H of thecomposite system of the imaging optical system 20 to the medium 6 to bescanned, N is the number of the reflecting surfaces of the deflector 3and 2W is the effective scanning width on the medium 6 to be scanned. Inthis case, a ghost image Pg is formed outside the distance W from theoptical axis C to the end of the effective scanning width and does notappear within the effective scanning width on the medium 6 to bescanned. Also, if a suitable light-intercepting plate is installed forintercepting the light beam Le of the ghost image Pg, the ghost imagecan be completely eliminated.

For example, where the deflector 3 is a rotatable polygon mirror havingN reflecting surfaces, N being 8, the effective scanning width W is 100mm, and the distance D from the image side principal point H of theimaging optical system 20 in the deflecting plane of the light beam Leto the medium 6 to be scanned is 300 mm, the relation that α<70.90 maybe adopted.

Thus, the scanning optical system of the present invention in which aghost image is eliminated imposes a predetermined limitation upon theangle which the optical axis of the light beam incident on the deflectorforms with the optical axis of the imaging optical system to the mediumto be scanned so that the ghost image always is stationary and outsidethe effective scanning width. Thus, the invention can prevent the ghostimage from appearing as an undesirable image on the surface of themedium to be scanned.

What is claimed is:
 1. A scanning optical system for scanning a surfaceand for preventing formation of a ghost image on said surface within theeffective scanning area that is scanned, said systemcomprising:deflector means having a plurality of reflecting surfaces; alight beam supplying optical system for forming a linear light beamimage near one of said reflecting surfaces of said deflector meanspositioned to deflect a light beam toward the surface; and imaging meanshaving different powers in orthogonal directions and rendering said onereflecting surface of said deflector means and the surface to be scannedin optically conjugate relation with each other in a plane perpendicularto the deflection plane formed by the light beam image as deflected bysaid deflector means; wherein the angle α which the optical axis of saidimaging means forms with the optical axis of said light beam supplyingoptical system satisfies: ##EQU1## where N is the number of reflectingsurfaces of said deflector means, W is one half of the width of theeffective scanning area, and D is the spacing between the principalpoint of said imaging means, which is adjacent to the surface to bescanned, and the surface to be scanned.
 2. A scanning optical systemaccording to claim 1, wherein said imaging means has f.θcharacteristics.
 3. A laser beam recording apparatus comprising:a laserslight source; a polygon mirror for deflecting the light beam from saidlaser light source, said mirror having a plurality of reflectingmirrors; a first imaging optical system for imaging the light beam fromsaid laser light source as a linear image near one of the reflectingsurfaces of said polygon mirror, and linear image being one whichextends in a direction perpendicular to the direction of the rotationalaxis of said polygon mirror; and a second imaging optical systemdisposed between said polygon mirror and a photosensitive medium and forimaging the light beam as deflected by said polygon mirror on thephotosensitive medium, said second imaging optical system havingdifferent powers in orthogonal directions so that the linear imageformed near said one of the reflecting surfaces of said polygon mirroris imaged on the photosensitive medium as a beam spot; wherein the angleα which the optical axis of said first imaging optical system forms withthe optical axis of said second imaging optical system satisfies:##EQU2## where N is the number of reflecting surfaces of said polygonmirror, W is one half of the width of the effective scanning area on thephotosensitive medium, and D is the spacing between the principal pointof said second imaging optical system, which is adjacent to thephotosensitive medium, and the photosensitive medium.
 4. A laser beamrecording apparatus according to claim 3, wherein said second imagingoptical system has f.θ characteristics.